High-frequency oscillator



March 16, 1954 W. W HARMAN HIGH-FREQUENCY OSCILLATOR Filed Aug. 30 1950 b\ NNW INVENTR I HAIR/WAN ATTORNEY mx .hl

mm. mi@ N Patented Mar. 16, 1954 U N IlED f. PATE l412,672,571

L"'-HIGH:FREQUENCY OSCILLATR Willis Wi Harman; Gainesville,` f Fla., y"assignmisto The Board-of"` Trustees of'The Leland-Stanford -J unionv University, StanfordUniversityfCaliL Application' August 30, 1950;` Serial No? A1f823l57 6 .Glaima This invention elatesto improvementsfin sysi tems for generating high-frequencyelectromag- 4netic Wave energy; and particularly toy oscillators utilizing the principle "offth' travelling Wave'aniplien wherein a stream lof--eelectrons'is -rnacle v to exchange energy with aivvave propagating fats'ubstantially the electron*velocity,' throughout an f 4extended interaction fspace. "Prior Iart''oscillators for opera-tion at centimeter'wavelength's have had the disadvantage of being tunable 1 only through i a relatively narrow frequency rangeg'ior'exaniple a reflex klystron"ofthef-deformable cavity resonator type mav'fbe'tuned over anhand Aof'about plus or minus 15-pereent. vl `Acco'rdin'gly, one of the i principal'objects of `the-presentinvention is jfto' provide high frequency"os'cillatorsl'which are tunable throughout relatively fwide' frequencybands. VWhile itis knownthattravelling wave tubes "maybe made tol oscilla't'e"Without-theT provision 'of any external feedback"/meansffandin -fa'ot I oscillate spontaneously' unless something-- is =done heretofore to 'control theffrequency' :ci oscillation to a practical extent. The tube IWill'tend"tofos'eillate` at one'or more ofa-'nuinbervof 'discretefrequences, but not at otherf'frequencies. "Theispecilc frequencies'at Whichlf-oseillation can'f-'oecur will depend upon the electricallen'gth lof the wave "propagating structure; and Aare"5characteristic A'of a particular tube.v Previous attempts to vary'ithe-v travelling wave tubeoscillators Whichliricludead-- f justable vmeans for freectively'terminating 'the Wave propagating structurewithout substantial l"conversion of the longitudinalwaves into spurious modes.

The invention will be desciibedlvvith'referenceV tov the accompanying'drawing; Whereirr`-thei single figure is a longitudinalcross section V4of a' present- 1y preferred embodimentY thereof.

f The travelling wave tubefin the illustrated'syspropagating helixf 3 ,e and "an 'electronk collector 'electrode 5 enclosed-*in arr-evacuated@envelopeV i "off dielectric' maaterial-lsuch asf'qua'rtz. "`The"-el*ecfftronl-fg-un Ifincludes a :cathode lil-'focussingielecf arranged'fingknown manneifitoi-precluceiandfdirect a beam; of 'zelectronsltravellingffrv-ithin f-andiialfong the axis of the helix" Sitothecol-leetont.Sflhe cathode S includes a heater adaptedfftohe feuergized from fl l'ani external lsoulrcesfsuchs-fas yaihat A directcurrent'sourceisucwas afbattery'l @may be connected tomaintaithefoeussingiaceelerating and collectinglelectrdesat thefre'duired potentials With respectltoiea'chethenrand hef-cathode. Typical voltagerfoithevariousielectrodes are:

Cathode :Zero-,1 iongrouno. Focussing ringfll` 1-45 Focussing .ring` :i3 `=-+3000 i Accelerator al 5----- .a 0 :to 42H00 Collector 5 f Ortoa'DOO 'The voltages at" the aceleratorand-1lectorie1ec theretothrough a'wi're 25.

The helix 31is a conductiveWin'dingbf'uniform tions 2l and 29 respectively. ^"These"tran"sitions may be'identicaLeach consistingof a'hollovvi'tubular .conductor 3 l 4provided'.ivvith a helic'altslot" 33.

Astantially equalto that bf'fthe'helix;""at the` :point Where the helix is.connectedtoi'the4 'transition sec- .parallel tothe axisofth tubular'memberl'toward the end remoteffrom the; helix.

I A tubular outerl conclusioni2"surrounds"the travelling wavetube '.'throughoutsubstantially'its 'entire length, extending 'viellibeyond'theends"of the transitionV sectionsZlandZS; :and isof relatively large' diameter so asit" be'radiallyLspac'ed any radio` frequency elds 'associated'-vvithfthe Waves travelling onthe helix 3. """Ih` middle portion of the conductor'garoundthehlixfunctions' o'nlvas'v protective 1 cover andi mechanical v support, and 'may be `oinittediii. desired.

` A short circuiting plugsprovided" near "one A A "end of thetube, -extendingrradiajllyUbetween"the Itcm comprlses an ielectron Agunva slcwfwaveaw.-

outer conductor lSil-and the end ofthe transition impedance-aramiddirequencynecnnection-iibtneen acvacvi the conductors 32 and 3| without requiring an actual metallic connection through the wall of the quartz envelope 1. The outer wall of the plug 35 may be in contact with the outer conductor 32. An output coupling loop 34 is provided near the plug 35, and is connected to and may be supported by a coaxial line 36 extending to any suitable load or utilization device, not shown.

A longitudinally movable short circuiting plug or tuning plunger 31 is provided at the other end of the tube, beyond the transition section 29. The tubular conductor 3| at this end is extended as shown at 39 to allow for substantial longitudinal travel of the plunger 31 through a region beyond the point where the pitch of the slot 33 becomes infinite. The plunger 31 is preferably of the non-contacting or choke type, both at the conductor 3| and the outer conductor 32, and is supported on a piston member 4| slidable longitudinally within the conductor 32. To provide for convenient adjustment of the plunger 31, the piston 4| may be coupled through a leadscrew 43 and gears 45 or equivalent means to a crank 41.

Any electrical disturbance will initiate a plurality of waves of different frequencies, each of which will propagate along the helix to one of the transition sections 21 or 29. The mode of propagation is one in which the electric field vectors extend longitudinally of the helix, being curved but lying substantially within planes which include the axis of the helix. This mode is referred to hereinafter simply as a longitudinal, or longitudinal electric mode.

As each wave reaches one of the transition Y sections, it is confronted with the variable pitch slot 33 and its electric field vectors are twisted toward a plane perpendicular to the axis. The longitudinal mode is gradually transformed into a radial or TEM mode, with the electric vectors extending radially between the tubular conductor 3| and the'outer conductor 32 as in the principal mode of an ordinary coaxial transmission line. Owing to the continuous nature of the transformation, substantially all of the longitudinal mode field is transformed into the radial mode, without generation of any spurious or undesired modes.

The Wave travelling in the substantially pure TEM mode is effectively reflected by the shorting device, and travels back along the transition section, which reconverts it to the longitudinal mode for propagation along the helix to the other end of the structure. Thus the transition sections 21 and 29, together with the snorting devices 35 and 31 and the adjacent portions of the outer conductor 32, cooperate to act as reectors at each end of the helix 3. When the wavelength is such that the electrical distance between these reflectors is an integral number of half wavelengths, the waves travelling in opposite directions are phased so as to set up a standing wave, i. e. the helix 3 and the reectors act as a resonator.

The wave component travelling in the same direction as the electron beam will intereact therewith, providing that the electrons are travelling at about the same velocity as the wave, and cause the stream to form into bunches. As the bunches travel with the Wave, some of the kinetic energy of the electrons is converted into Wave energy, causing the Wave to increase in amplitude as it travels along the helix from left to right as shown in the drawing. The amplified Wave is reectedat the plunger 31 and returned.

substantially without attenuation to the plug 35, where it is again reflected to travel in energy exchanging relationship with the electron stream.

Thus oscillations will build up to a point where the power given up by the wave, by losses in the tube and by extraction through the output coupling loop 34, is equal to the power delivered to the wave by the electron stream.

The helix 3, like most slow wave propagating devices, is more or less dispersive, i. e. the velocity of wave propagation depends upon the frequency of the wave being propagated. For the purpose of the present invention, it is preferable that the helix be designed to be as dispersive as is possible consistent with other factors such as reasonable gain per unit length. To produce oscillation at a selected frequency, the voltage dividers 2| and 23 are adjusted to make the velocity of the electrons conform approximately to the propagation velocity in the helix of waves of the desired frequency. The position of the plunger 31 is then adjusted by means of the crank 41 to make its electrical distance from the plug 35 exactly an integral number of half wavelengths at said frequency.

It will be apparent that there may be a number of possible positions for a given frequency, for example those corresponding to I9, 20, and 2| half wavelengths; any such position is suitable. Coarse adjustment to any frequency within a very wide band may be effected by variation of the accelerating voltage by the voltage divider 2|, and fine adjustment to an exact frequency within the relatively narrow band corresponding to a particular electron velocity may be made by moving the tuning plunger 31. One advantage of this is that the frequency may be changed rapidly and conveniently, making the system c particularly suitable for use as a signal generator, or as a local oscillator in a microwave superheterodyne radio receiver, for example.

Since many changes could be made in the above construction and many apparently widely different embodiments of this invention could be made without departing from the scope thereof, it is intended that all matter contained-in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

l. An oscillator for generating high frequency electromagnetic wave energy, comprising a wave propagating structure adapted to conduct travelling electromagnetic waves at a velocity which is slow compared to the velocity of light, means for producing and directing a stream of electrons along said structure in energy exchanging relationship with waves travelling in one direction therealong, wave reflector means at the end of said structure for reversing the direction of travel of waves arriving at said end, and means coupled to said structure for conveying electromagnetic Wave energy therefrom.

2. A high frequency device comprising a wave propagating structure adapted to conduct electromagnetic waves at a velocity which is low compared to the velocity of light and in a longitudinal electric mode, and refiectors at the ends of said structure, each of said reflectors comprising transition means for converting the longitudinal electric mode to a radial electric mode, said transition means including an inner conductor connected to one end of said structure and an outer condutor surrounding and radially aevas'n spaced from said inner conductor, and means for short circuiting said outer conductor to said inner conductor at a point remote from the end of said Wave propagating structure.

3. A high frequency device including a wave propagating structure of a type adapted to conduct electromagnetic waves in a mode wherein the electric eld vectors extend longitudinally in the direction of propagation, transition means for converting the longitudinal electric niode to a circumferentially uniform radial electric mode, said transition means including an inner conductor connected to the end of said structure and an outer conductor surrounding and radially spaced from said inner conductor, and a short circuiting plug between said inner and outer conductors.

4. A high frequency device comprising a ccnductive helix adapted to conduct electromagnetic Waves at a velocity which is low compared to the velocity of light and in a longitudinal electric mode, and reflectors at the ends of said helix, each of said reilectors comprising a tubular conductor having a helical slot which increases continuously in pitch from that of said helix at the point of connection thereto to substantially iniinite pitch at a second point on said conductor, a conductive sleeve surrounding and radially spaced from said tubular conductor, and means for short circuiting said sleeve to said tubular conductor at a point remote from the end of said helix.

5. A high frequency oscillator, including a conductive helix, an electron gun at one end of said helix and a collector electrode at the other end of said helix; terminating devices connected to the ends of said helix, said terminating devices each comprising means for converting waves travelling in a longitudinal electric mode on said helix to waves of a substantially circumferentially uniform radial electric mode, said means including a tubular conductor connected to the end of said helix, a conductive sleeve surrounding and spaced radially from said tubular conductor, and a short circuiting plug between said tubular conductor and said sleeve at a point spaced from the end of said helix, where the electric ield is radial between said tubular conductor and said sleeve; a coupling loop adjacent the short circuiting plug of one of said terminating devices, and a transmission line connected to said coupling loop and adapted to be connected to a utilization device.

6. A high frequency oscillator, including a conductive helix, an electron gun at one end of said helix and a collector electrode at the other end of said helix; terminating devices connected to the ends of said helix, said terminating devices each comprising a tubular conductor provided with a helical slot which has a pitch increasing from substantially that of the helix at its point of connection thereto to a substantially infinite value toward the other end of said tubular conductor, a conductive sleeve surroundn ing and spaced radially from said tubular conductor, and a short circuiting plug between said tubular conductor and said sleeve at a point further from said helix than the place Where the pitch of said slot becomes substantially infinite; a coupling loop adjacent the short circuiting plug of one of said terminating devices, and a transmission line connected to said coupling loop and adapted to be connected to a utilization device.

WILLIS W. HARMAN.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,122,538 Potter July 5, 1938 2,278,210 Morton Mar. 31, 1942 2,521,760 Starr Sept. l2, 1950 2,559,581 Bailey July 10, 1951 2,637,775 Lund May 5, 1953 OTHER REFERENCES Lund: R. C. A. Review, vol. II, No. 1, March 1950, pp. 13B-142.

I-Iollenberg: Bell System Technical Journal, pages 52-58, January 1949. 

